The overall goal of this research program is to advance understanding of how infants acquire flexibility in motor skill. Functional skills require flexibility because everyday life continually poses new problems that demand new solutions. Even high practiced actions such as sitting, standing, crawling, and walking must be modified on a moment to moment basis to cope with changing biomechanical constraints. The proposed studies will examine the processes and mechanisms by which infants learn to cope with novel situations and the circumstances under which they generalize what they have learned. The emphasis on flexibility shifts the core of this research from descriptions of motor development to central issues in psychology such as learning, memory, and the nature of developmental processes. Specific aim 1 is to examine online adaptation to altered biomechanical constraints. The proposed experiments will identify the variety of strategies infants use to cope with a novel perturbation to balance control, the relative effectiveness of each strategy and the role of experience in facilitating or impeding adaptation. Specific aim 2 is to investigate the exploratory movements that guide locomotion. The proposed experiments will examine how access to visual information about the feet and ground affects infants' search for further information. Specific aim 3 is to test learning and transfer of context- specific cues for guiding locomotion. The proposed experiments ask whether infants can learn to associate visual cues for surface rigidity and friction with the consequences for locomotion. Specific aim 4 is to describe the quantity and context of everyday locomotor experience. Four diary studies will track infants' active and passive experience with stance and locomotion from birth through several weeks after walking onset. Together, this research provides a comprehensive program for investigating how infants cope with varying body in an ever-expanding world. The project will yield a clearer picture of the joint roles of learning and development in achieving flexibility. In addition, results may have practical applications for promoting new skills in typically developing infants and children with motor impairments.